Packaging of microelectronic sensors, such as MEMS sensors, presents several problems that do not usually arise in connection with standard electronic circuit packaging. For instance, monolithic integration of sensor with sensor electronics tends to incur costly redesign of the overall circuit as design rules for the electronics evolve. Accordingly, hybrid integration approaches are often considered for sensor packaging. For example, separate electronics and sensor chips can be placed in a pre-molded package, wire bonded to each other and to the package, and overfilled with a gel. A lid having an aperture can be used to close the package, where the aperture in the lid provides environmental access to the sensor. Such an assembly has the advantage of placing the electronics and the sensor in close proximity to one another. Such an arrangement can be of particular importance in the field of capacitive sensors, where the electrical properties of the interconnections can introduce significant errors into the sensor readings. However, the overfilling gel is also exposed to the environment. Thus contaminants (e.g., water) can be absorbed by the gel and eventually reach the electronics chip, thereby causing corrosion of the chip traces. This problem could be alleviated by molding the package directly to the hybrid assembly, thereby reducing the gel volume or even eliminating it entirely. However, such molding processes tend to impose excessive mechanical stress on the sensor chip, and are thus often inapplicable (e.g., for mechanical sensors such as pressure sensors).
In view of such difficulties, several other sensor packaging approaches have been considered in the art. For example, in U.S. Pat. No. 6,140,144, a sensor unit is flip chip bonded to a substrate having a through hole, thereby providing access from the environment to the active area of the sensor while sealing off the rest of the sensor unit from the environment. In U.S. Pat. No. 6,891,239, a sensor is bonded to one side of a substrate and an electronics chip is bonded to the other side of the substrate. Electrical connections between the sensor and the electronics chip are made through the substrate. In U.S. Pat. No. 6,732,588, a sensor is formed by vertically bonding two chips together. In U.S. Pat. No. 6,927,482 a premolded package completely surrounds the electronics chip, while leaving a premolded cavity for the sensor chip, which is connected to the electronics chip via the leadframe. This approach protects and isolates the electronics chip from the sensor chip but also entails long connections between the sensor chip and the electronics chip. A similar approach, in which the electronics chip is covered by a filler, is shown in U.S. Pat. No. 4,838,089.
However, these approaches do not provide a complete answer to the above-identified sensor packaging problems. Accordingly, there is a need in the art for sensor packaging providing an improved combination of 1) low mechanical stress on the sensor; 2) access from the environment to the sensor active region; 3) isolation of all parts of the sensor other than the sensor active region from the environment; and 4) minimal interconnection length between the sensor and sensor electronics to reduce parasitics.